This study has 3 major objectives: a) to determine toxicities and the maximum tolerated dose of the non-immunosuppressive cyclosporin D analogue, SDZ PSC 833 (PSC), as a more potent and less toxic inhibitor of P-glycoprotein; b) to assess the toxicities of the combination of etoposide and PSC, and derive a dose modification factor for etoposide, and c) to study the effects of PSC on the pharmacokinetics of etoposide.